1. Field of the Invention
The present invention relates to the field of data storage devices. More particularly, the present invention relates to a magnetic recording channel front-end having an increased channel bandwidth without a degraded signal-to-noise ratio (SNR) performance.
2. Description of the Related Art
The readback bandwidth of conventional magnetic recording channel front-end depends on the impedance of the components forming the channel front-end, i.e., a magnetoresistive (MR) or giant magnetoresistive (GMR) element, an interconnect and readback electronics (i.e., readback amplifier). Until recently, the front-end readback bandwidth requirements were sufficiently low that the bandwidth provided by conventional front-end techniques presented no problems. Nevertheless, magnetic recording data rates have increased so that the readback bandwidth provided by a conventional channel front-end now limits channel transfer.
One approach for increasing the channel bandwidth is by decreasing the limitations on the so-called extrinsic bandwidth, that is, the bandwidth limitation that is caused extrinsic to the read electronics (not including the bandwidth limitations of the readback amplifier), is to terminate the read interconnect by an impedance that is substantially equal to the characteristic impedance of the interconnect. This, however, destroys the signal-to-noise ratio performance of a channel front-end.
To achieve a better defined interconnect, a microstrip connection between the head and readback amplifier has been proposed to replace a twisted-pair wire connection, which provides an ill-defined interconnection. Such a microstrip connection includes two co-planar flat signal conductors formed on a thin sheet of a non-conductive dielectric carrier material having a ground-plane backing, and becomes a transmission line at high frequencies. The characteristic impedance Z0 of a twisted pair, as is in wide-spread use in magnetic recording, is 75-85 Ohms. The characteristic impedance Z0 of a microstrip interconnect depends on the width of the conductors, the spacing between the conductors, and the thickness and dielectric constant of the carrier material. Practically speaking, the characteristic impedance Z0 of a microstrip interconnect in the range of 35-85 Ohms.
There are four configurations for biasing an MR (GMR) element and for reading out, or reading back, a signal: (1) a current bias with current-sensing readback, (2) a voltage bias with current-sensing readback, (3) a voltage bias with voltage-sensing readback, and (4) a current bias with voltage-sensing readback. Because the frequency characteristics of a channel front-end are not sensitive to the particular biasing technique used, only the frequency characteristics of the readback technique used need be considered.
FIG. 1 shows a schematic diagram of a conventional single-ended input, current-sensing readback amplifier configuration 100. In FIG. 1, an MR element Rmr that is located in a head 101 is connected to a current-sensing readback amplifier 102 through an interconnect 103. Interconnect 103 has a characteristic impedance Z0=R0.
Current-sensing readback amplifier 102 includes an npn input transistor 104 and a load resistor RL. Conductor 103a of interconnect 103 is connected to the emitter of transistor 104. Conductor 103b of interconnect 103 is connected to circuit common, or ground. The base of transistor 104 is connected to a bias voltage Vbias, and the collector of transistor 104 is connected to a power supply voltage V+ through load resistor RL. The output vo of readback amplifier 102 appears across load resistor RL.
The input impedance Zin for a current-sensing type of readback amplifier, shown in FIG. 1, is the differential emitter resistance of input transistor 104 and is given by Zin=re=kTqI, where k is Boltzmann""s constant, T is the absolute temperature, q is the charge of an electron, and I is the emitter current of transistor 104. Current I provides the bias current for MR element Rmr, which is typically 5-12 mA. Thus, the input impedance Zin is typically 2-5xcexa9 at room temperature.
FIG. 2 shows a schematic diagram of a conventional single-ended input, voltage-sensing readback amplifier configuration 200. In FIG. 2, an MR element Rmr that is located in a head 201 is connected to a voltage-sensing readback amplifier 202 through an interconnect 203. Interconnect 203 has a characteristic impedance Z0=R0.
Voltage-sensing readback amplifier 202 includes an npn input transistor 204, an input coupling capacitor 205, current sources 206 and 207, an npn cascode transistor 208, and a load resistor RL. Conductor 203a of interconnect 203 is connected to the base of transistor 204 through coupling capacitor 205. Conductor 203b of interconnect 203 is connected to circuit common, or ground. Current source 206 supplies bias current Ibias to MR element Rmr. Current source 207 supplies current to the base of transistor 204. The emitter of transistor 204 is connected to circuit common, or ground. The collector of transistor 204 is connected to the emitter of cascode transistor 208. Cascode transistor 208 is used for eliminating the Miller capacitance at the input of readback amplifier 202. The collector of transistor 208 is connected to a power supply voltage V+ through load resistor RL. The output vo of readback amplifier 202 appears across load resistor RL.
The input impedance Zin for a voltage-sensing type of readback amplifier is given by Zin=xcex2re, where xcex2 is the current gain of transistor 204, and re is the emitter resistance of transistor 204. Input impedance Zin is typically 500-1000xcexa9. Input capacitance 205 is approximately 3 pF and is given by 1/2xcfx80ftre, where ft is the transition frequency of transistor 204, which is typically around 10 GHz.
The differential-input versions of the readback amplifiers of FIGS. 1 and 2 are not shown, but have input impedances that are respectively twice the values described above for the single-ended input readback amplifiers. An MR (GMR) element connected to a readback amplifier through an interconnect that has a characteristic impedance Z0 xe2x80x9cseesxe2x80x9d an impedance Zi that is given by                                           Z            i                    =                                    Z              0                        ⁢                                                            Z                  in                                +                                  j                  ⁢                                      xe2x80x83                                    ⁢                                      Z                    0                                    ⁢                  tan                  ⁢                                      xe2x80x83                                    ⁢                                      γ                    ⁢                    1                                                                                                Z                  0                                +                                  j                  ⁢                                      xe2x80x83                                    ⁢                                      Z                    in                                    ⁢                  tan                  ⁢                                      xe2x80x83                                    ⁢                                      γ                    ⁢                    1                                                                                      ,                            (        1        )            
where xcex3=xcfx89/xcexd, xcexd is the transmission line velocity (≅200 km/s for xcex5r=2.25) and 1 is the length of the interconnect, which is typically 5 cm for a 3.5 inch drive.
For a current-sensing readback amplifier configuration, the input impedance Zin=kT/qI1 less than  less than Z0. For xcex31 less than  less than xcfx80/2, Zi approaches                                           Z            in                    +                      j            ⁢                          xe2x80x83                        ⁢                          Z              0                        ⁢            tan            ⁢                          xe2x80x83                        ⁢                          γ              ⁢              1                                      ≃                              r            e                    +                      j            ⁢                          xe2x80x83                        ⁢            ω            ⁢                          xe2x80x83                        ⁢                          Z              0                        ⁢                                          1                ν                            .                                                          (        2        )            
Thus, when a current-sensing readback amplifier configuration is used, the interconnect behaves like an inductor Z01/xcexd that is in series with readout element Rmr. The transfer characteristic due to the interconnect is then                                           v            o                                v            i                          =                                            R              L                                                      R                mr                            +                              r                e                                              ⁢                      xe2x80x83                    ⁢                                    1                              1                +                                  j                  ⁢                                      xe2x80x83                                    ⁢                  ω                  ⁢                                      xe2x80x83                                    ⁢                                                            Z                      0                                                                                      R                        mr                                            +                                              r                        e                                                                              ⁢                                      1                    ν                                                                        .                                              (        3        )            
Equation (3) shows that the channel bandwidth of a current-sensing readback amplifier configuration is restricted by the interconnect, and is further reduced as Z0 or 1 increases, or as xcexd decreases.
For a voltage-sensing readback amplifier configuration, Zin=xcex2re greater than  greater than Z0. For xcex31 less than  less than xcfx80/2, Zi approaches                                                         Z              0                        ⁢                          Z              in                                                          Z              0                        +                                          Z                in                            ⁢                              xe2x80x83                            ⁢              j              ⁢                              xe2x80x83                            ⁢              tan              ⁢                              xe2x80x83                            ⁢                              γ                ⁢                1                                                    .                            (        4        )            
Thus, when a voltage-sensing readback amplifier configuration is used, the interconnect behaves like a capacitor 1/xcexdZ0 that is in parallel with readout element Rmr and the input impedance of the readback amplifier. The transfer characteristic due to the interconnect is then                                           v            o                                v            i                          =                              A                          1              +                              j                ⁢                                  xe2x80x83                                ⁢                                                      tan                    ⁢                                          xe2x80x83                                        ⁢                                          γ                      ⁢                      1                                                                            Z                    0                                                  ⁢                                  R                  mr                                                              ⁢                      xe2x80x83                    ≃                                    A                              1                +                                  j                  ⁢                                      xe2x80x83                                    ⁢                  ω                  ⁢                                      xe2x80x83                                    ⁢                                      1                                          ν                      ⁢                                              xe2x80x83                                            ⁢                                              Z                        0                                                                              ⁢                                      R                    mr                                                                        .                                              (        5        )            
Equation (5) shows that the channel bandwidth for a voltage-sensing readback amplifier configuration is similarly restricted by the interconnect, and is further reduced for large 1, small xcexd and small Z0.
When an interconnect is terminated with the characteristic impedance Z0 of the interconnect at the input to the readback amplifier, there are no bandwidth constraints because the read element merely sees an impedance Z0. Terminating the interconnect in its characteristic impedance Z0 can be done for a current-sensing readback amplifier configuration by inserting a resistor R8=Z0xe2x88x92Zin in series with the input impedance of the readback amplifier. For a voltage-sensing readback amplifier configuration, the interconnect can be terminated by placing a resistor having a value of ZinZ0/(Zinxe2x88x92Z0)=Z0xcex2re/(xcex2rexe2x88x92Z0≅Z0 in parallel across the input of the readback amplifier. This, however, results in an unacceptable decrease in the signal-to-noise ratio (SNR) performance for the electronics.
For a current-sensing readback amplifier configuration, the SNR performance decreases from                     SNR        =                              ν            i                                              4              ⁢                              kT                ⁡                                  (                                                            r                                              bb                        xe2x80x2                                                              +                                                                  1                        2                                            ⁢                                              r                        e                                                                              )                                                                                        (        6        )            
to                     SNR        =                                            ν              i                                                      4                ⁢                                  kT                  ⁡                                      (                                                                  r                                                  bb                          xe2x80x2                                                                    +                                                                        1                          2                                                ⁢                                                  r                          e                                                                    +                                              R                        s                                                              )                                                                                .                                    (        7        )            
In Equations (6) and (7), xcexdi is the readback signal of the MR (GMR) element, and rbbxe2x80x2 is the base bulk resistance of the input transistor, which is typically 1-5xcexa9 for MR readback amplifiers. For typical values of T=300 K, rbbxe2x80x2=1xcexa9, re=5xcexa9 and Rs=70xcexa9, the SNR performance for a current-sensing readback amplifier configuration degrades by a factor of 4.6.
For a voltage readback amplifier configuration, the SNR performance decreases from                     SNR        =                              ν            i                                              4              ⁢                              kT                ⁡                                  (                                                            r                                              bb                        xe2x80x2                                                              +                                                                  1                        2                                            ⁢                                              r                        e                                                                              )                                                                                        (        8        )            
to                     SNR        =                                            ν              i                                                      4                ⁢                kT                ⁢                                  {                                                                                    (                                                                              r                                                          bb                              xe2x80x2                                                                                +                                                                                    1                              2                                                        ⁢                                                          r                              e                                                                                                      )                                            ⁢                                              xe2x80x83                                            ⁢                                                                        (                                                                                                                    Z                                0                                                            +                                                              R                                mr                                                                                                                    Z                              0                                                                                )                                                2                                                              +                                                                  R                        mr                        2                                                                    Z                        0                                                                              }                                                              .                                    (        9        )            
For typical values of T=300 K, rbbxe2x80x2, =1xcexa9, Re=5xcexa9, Z0=75xcexa9 and Rmr=75xcexa9, the SNR performance for a voltage-sensing readback amplifier configuration degrades by a factor of 2.6.
What is needed is a way to decrease the bandwidth limitations of a magnetic recording channel front-end that are caused extrinsic to the read electronics without degrading the SNR performance of the channel front-end.
The present invention provides a way to decrease the bandwidth limitations of a magnetic recording channel front-end that are caused extrinsic to the read electronics without degrading the SNR performance of the channel front-end.
The advantages of the present invention are provided by a magnetic recording channel front-end for a magnetic storage system, such as a disk drive, that includes a magnetoresistive element, an interconnect and a readback amplifier. The interconnect has a characteristic impedance, and couples the magnetoresistive element to the readback amplifier. The readback amplifier includes a gain stage and an active termination. The gain stage has an input with an associated input impedance that is connected to the interconnect. According to the invention, the active termination is coupled to the input of the gain stage, such that the input impedance of the readback amplifier, which is formed by a combination of the impedance associated with the gain stage and the active termination, is substantially equal to the characteristic impedance of the interconnect. Additionally, the gain stage generates a first noise signal that has a first magnitude appearing at an output of the gain stage. The active termination generates a second noise signal that has a second magnitude that also appears at the output of the gain stage. According to the invention, the second magnitude is significantly less that the first magnitude, thereby the signal-to-noise ratio performance of the magnetic recording channel front-end is not adversely affected.
When the gain stage is a current-sensing gain stage, the active termination provides positive feedback from the gain stage to the input of the gain stage. When the gain stage is a voltage-sensing gain stage, the active termination provides negative feedback from the gain stage to the input of the gain stage.